MadSci Network: Physics

Re: effect of temperature on light

Date: Fri Nov 20 08:05:05 1998
Posted By: Bob Novak, Other (pls. specify below), Sr Process Research Engineer, Carpenter Technology
Area of science: Physics
ID: 910580999.Ph

Hi Ben,

Sounds like an interesting project.  I checked with a friend who does 
surveying work.  The electronic distance meter does have adjustments for 
temperature and pressure.  For normal surveying work they are not used.  
The adjustments are used for extremes of temperature or pressure, such as 
work above 10,000 feet in elevation.  Some surveying instruments make the 
adjustments automatically.

Why is the adjustment needed?  You are correct that laser light passes 
through clear air with very little interaction. The beam is only visible 
when dust or other particles are present to reflect the light back to your 
eye.  However, the speed of light is only a constant in vacuum.  The speed 
of light changes when it passes through a material, such as air. The ratio 
of the velocity of light in a vacuum to the velocity of light in a material 
is called the index of refraction.  The density (number of atoms or 
molecules per cubic meter) of air changes with temperature and pressure.  
It is the change in the density of the air, which changes its index of 
refraction, and changes the speed of light in air.  In the surveying 
equipment, the distance is measured by multiplying the speed of light 
(2.99792458 X 10 8 meters/second) by the time in seconds to get the 
distance traveled in meters.  If the speed of light changes, the accuracy 
of the distance measurement also changes. The time interval that needs to 
be measured is very short.  At 100 meters the time it takes the light to 
travel to the reflector and back is only 6 microseconds!

Have you ever looked at a hot object, such as a metal roof, on a sunny day? 
The air above the object seams to shimmer.  Some people call this heat 
waves.  The hot metal is heating the air above it and causing localized 
changes in the density of the air.  The differences in the index of 
refraction cause the light passing through the heated air to be diffracted 
or bent.  The diffracted light causes the shimmering effect we see with our 
eyes.  The heated air is acting like a lens.
Because the heating of the air is somewhat random, it is very difficult to 
correct for variations caused by heat waves.  Astronomers do the 
corrections by creating an artificial guide star with a laser beam.  The 
laser beam is visible because it reacts with a layer of sodium atoms in the 
upper atmosphere.  A computer looks at the image of the laser beam and 
bends the optical elements of the telescope to correct the shape of the 
laser beam.  Because the light from the stars follows the same path as 
the light from the guide star, the image of the starlight passing through 
the atmosphere is corrected for distortions caused by heat waves in the 

Setting up an experiment to measure the speed of light could be 
interesting.  You might try  using the laser range finder on the EDM to 
measure differences in transit time through different materials, if your 
dad lets you use his equipment.

Good luck with the project,
Bob Novak

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